The present invention relates to ink supply amount control for a printing press and, more particularly to an ink supply amount control method and apparatus which can suppress any density variation in a printing product with a small image area by intermittently stopping swing (ink feed operation) of an ink ductor roller.
Generally, the ink supply amount to a printing plate in a printing press is controlled by a gap amount between an ink fountain key and an ink fountain roller. FIG. 14 shows the main part of an ink supply apparatus (inker) provided in a printing unit for each color (ink color) in a rotary printing press. Referring to FIG. 14, an ink fountain 1 stores ink 2. An ink fountain roller 3 supplies the ink from the ink fountain 1 to the ink supply path. A plurality of ink fountain keys 4 (4-1 to 4-n) are juxtaposed in the axial direction of the ink fountain roller 3. An ink ductor roller 5 is arranged in the ink supply path to supply the ink from the ink fountain keys 4 to ink rollers 6. A printing plate 7 is attached to the outer surface of a plate cylinder 8. The ink is supplied from the ink rollers 6 including an ink distribution roller 6-1 and ink form rollers 6-2 to the printing plate 7.
In a four-color rotary printing press shown in FIG. 15, a printing unit 9 for each color individually has the ink supply apparatus shown in FIG. 15.
In the printing press having the above arrangement, the ink in the ink fountain 1 is supplied to the ink fountain roller 3 through the gap between the ink fountain keys 4 and the ink fountain roller 3 as the ink fountain roller 3 rotates. When the ink ductor roller 5 swings, the ink supplied to the ink fountain roller 3 is transferred to the ink ductor roller 5. The ink transferred to the ink ductor roller 5 is transferred to the ink distribution roller 6-1. The ink 2 transferred to the ink roller 6-1 is distributed by the ink rollers 6 and then supplied to the printing plate 7 through the ink form rollers 6-2. The ink supplied to the printing plate 7 is printed on a printing paper sheet through a blanket cylinder (not shown).
The gap amount (opening ratio of the ink fountain keys 4-1 to 4-n) between the ink fountain keys 4-1 to 4-n and the ink fountain roller 3 is set in accordance with the image area ratio in each of areas of the printing plate 7, which correspond to the ink fountain keys 4-1 to 4-n, respectively. For example, the set values of the opening ratios of the ink fountain keys 4-1 to 4-n are obtained in accordance with a preset “image area ratio—ink fountain key opening ratio conversion curve”, and the opening ratios of the ink fountain keys 4-1 to 4-n are adjusted. The value of the rotation amount (feed amount) of the ink fountain roller 3 is defined in advance. The opening ratios of the ink fountain keys 4-1 to 4-n and the feed amount of the ink fountain roller 3 are set for the printing unit 9 of each color.
In the ink supply apparatus in each printing unit 9, the ink ductor roller 5 reciprocally swings between the ink fountain roller 3 and the ink roller 6-1 to transfer the ink from the ink fountain roller 3 to the ink roller 6-1. The reciprocal operation (ink feed operation) is done by the same driving source as that of the printing press in synchronism with the rotation of the plate cylinder 8 (the rotation of the printing press), thereby obtaining a predetermined ink transfer amount. For example, the ink ductor roller 5 is reciprocally swung once by a driving cam that rotates once in synchronism with six revolutions of the plate cylinder 8.
Along with the recent increase in operation speed of printing presses, the balance between the ink supply amount to a printing paper sheet and the open/close of ink fountain keys becomes delicate. It is therefore difficult to stably supply ink. Especially, for a printing product with a low image area ratio (a printing product with a small image), ink in an excess amount is supplied into the ink supply apparatus, resulting in a density variation.
An ink feed apparatus disclosed in Japanese Patent Laid-Open No. 5-147200 (reference 1), the swing of the ink ductor roller is intermittently stopped to reduce the ink supply amount into the ink supply apparatus, thereby suppressing a density variation in a printing product with a small image. To intermittently stop the swing operation, for example, the number of revolutions of a rotary shaft that rotates coaxially with a driving cam that reciprocally swings the ink ductor roller is detected by a sensor. An air cylinder is actuated at a ratio corresponding to an integral ratio to the detected number of revolutions. With this operation, the ink ductor roller is forcibly pressed against (locked to) the ink roller side, thereby stopping the reciprocal operation of the ink ductor roller.
The ink feed apparatus described in patent reference 1, however, the operator checks the image of the printing product to be printed or the finished printing product and determines whether intermittent stop of the ink feed operation should be executed. Only experienced operators can make correct determination. If the ink feed operations are not thinned out when the number of times of ink feed must be decreased (when the ink feed operation must be intermittently stopped), or conversely, if the ink feed operations are thinned out when an operation in a normal state is necessary, normal printing products cannot be obtained. Alternatively, if a density variation occurs after the start of actual printing, the mode must then be switched to thinning-out operation to adjust the opening ratio of each ink fountain key. In this case, the amount of wasted paper increases, the operation takes long time, the load on the operator increases, the printing materials are wasted, and the operation efficiency becomes low.